Steady Flame Research Articles

Correlating concurrent-flow flame spread rates in different pressure and oxygen conditions: Ground experiments and comparisons with previous micro-, partial, and normal gravities experiments

Ambient pressure and gravity are important parameters in buoyant flow that governs upward flame spread process. Based on the concept of pressure modelling, this experimental study investigates extinction and upward flame spread process of a thermally-thin solid fuel in different pressure and oxygen conditions. Experiments are performed in a combustion chamber in air at different pressures (ranging from 10 kPa to 100 kPa) and different oxygen molar fraction environment (9–21 %). As pressure increases, different burning behaviors are observed: no ignition, partial flame spread, steady flame spread, and accelerating flame spread. Similar trend is observed as the ambient oxygen molar fraction increases. In partial pressure conditions (e.g., 25–50 kPa), flames exhibit characteristics that are typically observed in micro- and partial gravity environments: blue and dim. Flame spread rate and sample burnt length are deduced and compared between different pressure and oxygen levels. Overall, the burning intensity and the flame spread rate decrease with the decrease in ambient pressure and oxygen. The decrease in flame spread rate at reduced pressure is attributed to increase in flame standoff distance and decrease in convective heat transfer to the solid, whereas the decrease in flame spread rate in reduced oxygen molar fraction environment is attributed to decrease in flame temperature. Lastly, current and previous studies performed at different ambient environments are correlated using the concept of flame standoff distance (δf), which is estimated using the theoretical viscous boundary layer thickness (δv). It was found that approximating δf∼δvfor forced flow and δf∼1/3δv for natural flow can predict the flame spread rate reasonably well for data obtained in micro-, partial, and normal gravities, for a wide range of environmental conditions away from extinction limits.

Ultralean Combustion Mechanism of Methane–Air Swirling Premixed Flame

ABSTRACT This study investigated a methane – air premixed flame in a partially tapered swirl burner. Experimentally, we succeeded in forming almost steady ultralean flames with an equivalence ratio as lean as 0.465. On the other hand, a numerical simulation of the flame with detailed chemistry revealed that the flame head existed in a large recirculation zone under ultralean conditions. In addition, high-value peaks of temperature, heat-release rate (HRR), and concentrations of OH, O, and H radicals were formed at the flame head, enabling ultralean combustion. Subsequently, to identify the formation mechanism of these peaks, adiabatic swirl burner flames were simulated with and without the effect of thermal-diffusive imbalance (a Lewis number effect). The results show that a Lewis number of less than unity caused a temperature increase in the flame located in the recirculation zone owing to the thermal-diffusive imbalance, which increased the peak heights of the HRR and the three radicals. The results also showed that even in a flame unaffected by thermal-diffusive imbalance, the backward convective transport could accumulate CO and OH at the flame head thus enhancing the chemical reactions at the head. (185 words)

The effect of hydrogen enrichment on the conical premixed methane–air flame response and thermoacoustic modes coupling

This paper investigates the thermoacoustic dynamic responses of hydrogen-enriched laminar premixed conical methane–air flames under dual-mode coupling. Utilizing the level set method and the G-equation model, one meticulously derives the flame describing function (FDF) in relation to variations in hydrogen enrichment levels (ηH). This precisely derived FDF is then integrated into the low-order network model of the Rijke tube to analyze the thermoacoustic unstable modes of the system. Finally, dual-frequencies incoming flow velocity perturbations are reintroduced as inputs to obtain the flame response under thermoacoustic modes coupling. Results show that while keeping the unstretched steady flame aspect ratio constant, an increase in ηH not only raises the FDF’s cut-off frequency but also enhances the FDF gain within the nonlinear frequency region, leading to more unstable modes, especially high frequency modes within the Rijke tube system. Furthermore, the flame response is further altered under the excitation of dual unstable modes. When both modes are within the linear frequency region of the FDF, the flame response is co-controlled by the two modes, predominantly by the mode with a larger velocity perturbation amplitude, with weaker modes coupling leading to the additional frequency perturbation having a suppressive effect on the flame response at the original frequency. Conversely, when one or two modes are within the nonlinear frequency region of the FDF, the flame response is dominated by the lower-frequency mode, with higher nonlinear modes coupling allowing the additional frequency perturbation to both promote and suppress the response at the original frequency and also couple to produce a significant difference frequency response. Novelty and significance The novelty of this paper lies in the determination of the flame describing function (FDF) with varying hydrogen enrichment levels and the stability of thermoacoustic systems, and more significantly, conducting an in-depth and comprehensive investigation into the nonlinear dynamic responses of hydrogen-enriched flames to different types of dual-mode coupling perturbations. The dual-mode perturbations result in either attenuation or amplification of the flame response, depending on whether the corresponding frequencies lie within the linear or nonlinear frequency regions of the FDF, which innovatively incorporates the influence of the FDF phase and examines the responses at the difference frequencies generated by the coupling. This lays a foundation for further exploration into the mechanisms of modes coupling in hydrogen-enriched and other thermoacoustic systems.

Investigation on flame behaviors in the mesoscale channel of one sudden-expansion step during the preliminary stage of ignition

The dynamic flame behaviors at the preliminary stage of ignition in the mesoscale sudden-expansion channel are numerically investigated in this study under the isothermal condition (300 K) at walls by using a two-dimensional model without symmetric assumption at the centerline. It is found that the flashback velocity is dominated by the upstream channel height; nevertheless, the blowoff velocity is determined by not only the downstream channel height but also the flow recirculation behind the sudden-expansion steps. As the expansion ratio is sufficiently large, the flame could exist within a substantially wider range of inlet flow velocity. In addition, four types of flame behaviors are found at the expansion ratio of 2: (I) steady convex flame, (II) steady concave flame, (III) simple oscillating flame, and (IV) complex oscillating flame. Above the flashback velocity, the convex flame exists steadily. With the increase in flow velocity, the flame becomes concave to the upstream and is stabilized by the wall-quenching effect of sudden-expansion steps. If the flow velocity is further increased, the flame becomes unstable and oscillates periodically (simple oscillating flame) due to the interaction of flame and the symmetric flow field in the sudden-expansion channel, while the occurrence of complex oscillating flame at high flow velocities is attributed to the asymmetric flow pattern. The frequency of oscillating flames decreases with the increase in flow velocity.

A new experimental method to study the flame inhibition performance of low concentration halogenated gas

The minimum fire extinguishing concentration is a key index for the development of next generation chemical clean fire extinguishing gases, which is usually got by the cup-burner. The fire suppression performance of low concentration of chemical gas is hard to be understood by the available methods. In this work, the fundamental acknowledgement of steady flame surface attacking mechanism by halon functional group is newly discussed by conducting a series of bench scale tests with a cylindrical channel under a varied fluorinated gases. The characteristics of flame propagation under differentiated air atmosphere environmental conditions is obtained. The influence of fluorine-containing gas flow rate on flame propagation speed is clarified. A dimensionless relationship including atomic radius, bond length, polarization degree, molar mass and molar volume are proposed to describe the influence of low concentration of fluorine-containing gas on flame propagation speed. It provides an insight for the understanding of the flame inhibition performance of low concentration halogenated gas.

Seeing through fire with one pixel

Seeing through fire is a critical capability in various applications, such as fire rescue, industrial combustion monitoring, and scientific research. However, the intense electromagnetic radiation emitted by flames can saturate and blind conventional imaging systems, making it challenging to visualize objects or scenes obscured by fire. In this paper, we present a novel method for real-time, full-color through-flame imaging using structured illumination and single-pixel detection. By projecting a series of carefully designed light patterns onto the scene and measuring the backscattered light with a single-pixel detector, we can computationally reconstruct the obscured scene while effectively suppressing the flame's contribution to the image. The single-pixel detector's high dynamic range and sensitivity enable it to capture the weak backscattered signal without being overwhelmed by the flame's intense radiation. We demonstrate the method's effectiveness in several experiments, showcasing its ability to image static and dynamic scenes through both steady and turbulent flames at a frame rate of 15 Hz. Furthermore, we show that the proposed method can be extended to full-color imaging by using three single-pixel detectors with different color filters. The results highlight the potential of this approach for enhancing visibility in fire-related scenarios and other challenging imaging conditions. We believe that the integration of this technology into augmented reality systems could provide firefighters and other users with valuable real-time visual information, improving situational awareness and decision-making in critical situations. We also believe the method can be used for seeing through any self-luminous objects.

The burnt gas flow stability of limit flames in vertical tubes

Stability of the gas flows generated by steady near-limit flames propagating in vertical tubes is studied numerically. Basic scenarios of the burnt gas flow evolution are identified in relation to the thermal gas expansion parameter and the normal flame speed. It is shown that the realization of specific scenario essentially depends on the stagnation zone width as well as on the distance travelled by the gas from the flame front to the tube end. In particular, it is found that for sufficiently short distances, the burnt gas flows are stable provided that the stagnation zone width is less than half the tube diameter. Otherwise, an unstable flow evolution can lead to the appearance of recirculation domains and acoustic perturbations.

Steady laminar stagnation flow NH[formula omitted]-H[formula omitted]-air flame at a plane wall: Flame extinction limit and its influence on the thermo-mechanical stress and corrosive behavior of wall materials

The steady laminar stagnation flow flame of NH3-H2-air gas mixture stabilized at a plane wall is numerically investigated. Its interaction with the wall with the consideration of heat loss is the focus of this work. The numerical study of the combustion system is performed by using the full chemical mechanism and detailed transport model including the differential diffusion and Soret effect. The simulation of the solid mechanics is based on the theory of isotropic linear thermo-elasticity. With the numerical simulation, it will be discussed how the wall material would change the flame stability in terms of extinction limit, and how the combustion system such as mixture composition, flame strain rate, and pressure would vary the thermo-mechanical stresses in the solid wall and the corrosive behavior at the surface of the wall.

A Review on Micro-Combustion Flame Dynamics and Micro-Propulsion Systems

This work presents a state-of-the-art review of micro-combustion flame dynamics and micro propulsion systems. In the initial section, we focus in on the different challenges of micro-combustion, investigating the typical length and time scales involved in micro-combustion and some critical phenomena such as flammability limits and the quenching diameter.We present an extensive collection of studies on the principal types of micro-flame dynamics, including flashback, blow-off, steady versus non-steady flames, mild combustion, stable flames, flames with repetitive extinction, and ignition and pulsatory flame burst. In the final part of this review, we focus on micropropulsion systems, their performance metrics, conventional manufacturing methods, and the advancements in Micro-Electro-Mechanical Systems manufacturing.

Flame visualization and spectral analysis of combustion instability in a premixed methane/air-fueled micro-combustor

The present study experimentally explores the combustion instability characteristics of premixed methane/air flames in a planar micro-combustor by utilizing the high-speed camera and the noise data acquisition system. It aims to identify the sound pressure fluctuation of unstable flames and shed light on the relationship between flame structure and thermoacoustic instability under micro-scale conditions. Four types of flame structures, i.e., flame with repetitive extinction and ignition (FREI), weak flame, steady flame, and oscillating flame, are experimentally observed by varying operating parameters. Among them, the weak flame at low flow velocities and the oscillating flame at high flow velocities form thermoacoustic oscillation. For weak flames, the collision with the wall and the local deformation of the flame result in high-frequency harmonics. Varying the inlet velocity from 0.14 m/s to 0.15 m/s leads to the pressure fluctuation of the weak flame being reduced by 10 Pa, and the dominant frequency being changed from 125 Hz to 167 Hz. As for oscillating flame, the variation of the inlet velocity plays a decisive role in the sound pressure fluctuation. At the equivalence ratio of 0.7, an increase of 0.01 m/s in the inlet velocity reduces the pressure fluctuation by 4 Pa.

Soot emissions of steady and oscillatory candle flames

Soot particle emissions from steady flames have been extensively studied; however, less attention has been given to oscillating flames. This study aims to characterize the emission of soot particles from oscillatory flames generated by burning two bundles of candles (four candles per bundle) and compare them with a steady flame case. When two oscillating flames interact at varying separation distances, they exhibit two oscillatory modes and one steady mode, namely, in-phase (IP) oscillating flame, oscillation suppression (OS) flame, and anti-phase (AP) oscillating flame. Time-resolved flame images were captured using a digital camera, revealing that the OS mode represents steady flames, while the IP and AP modes represent symmetric and asymmetric oscillating flames, respectively. The flame height and the area were measured for both steady and oscillatory flames. The mean flame height remained nearly constant across all flame modes, whereas the mean flame area exhibited significant variations among each flame mode. Additionally, the diameter of soot particles was measured using the dynamic light scattering technique. The results indicate that steady flames produced the smallest soot particles (78 nm) compared to their counterparts in oscillatory flames. Furthermore, within the oscillatory flames, the symmetrically oscillating IP mode generated larger-sized soot particles (129 nm) compared to the asymmetrically oscillating AP mode (102 nm).

High-Resolution TDLAS Tomography of Gaseous Temperature and H2O Concentration in Steady Flames

High-Resolution TDLAS Tomography of Gaseous Temperature and H₂O Concentration in Steady Flames

Real-time temperature monitoring technology for dynamic combustion processes using dual-probe femtosecond CARS

We propose a fast responsive and non-invasive optical temperature measurement method which can be used to monitor the temperature of air-fed flames in real time. This method is based on femtosecond time-resolved coherent anti-Stokes Raman spectroscopy (fs-CARS). Two probe pulses with different wavelengths are employed in the CARS system to generate two CARS signals, and the intensity ratio of the two CARS signals has a definite numerical relationship with temperature and therefore can be used to rapidly measure the flame temperature. The temperature of methane/air swirl flames was monitored by the dual-probe fs-CARS thermometry with a sampling rate of 100 Hz. For the steady swirl flame, the measured average temperature in the inner recirculation zone is 1551 K with a standard deviation of 3.7%. While, at a higher methane/air equivalent ratio of 0.74, the flame cannot remain steady, and the temperature frequently jumps from around 1700 K to 2000 K, causing the vortex to expand and break. A significant advantage of the dual-probe fs-CARS thermometry is that the temperature of the flame can be obtained immediately during the measurement without any post-processing. Therefore, this method is a potential candidate for accurately monitoring the real-time temperature of turbulent combustion.

Safety aspects of Organic Rankine Cycles (ORC) with combustible working fluid and sub-ambient condenser pressure

ORCs are often designed with the lower pressure limited by the ambient pressure, to avoid air leaking into the working fluid. Safely lowering this limit will be beneficial for power-production efficiency. First, cycle operations with air inleak are investigated. Exemplary cases with n-pentane and benzene are used. The investigation combines tools from thermodynamics of cycle working fluids, pump operation and analysis and combustion science. It appears that pumps customarily used in ORC will forward only liquid, and air mixing into the working fluid at the high-pressure side will not be sufficient for flammable conditions. Next, combustion properties of the air/working fluid mixture in the condenser are evaluated. Transient zero-dimensional and one-dimensional steady state laminar premixed flames are applied for the modelling. Detailed chemical mechanisms from the CRECK and TDTVT groups are used for benzene and n-pentane. It is found that some substances, like pentane, will not make flammable mixtures within the conditions of the power cycle. However, fluids that at the relevant temperatures have saturation pressure much lower than the ambient, might give flammable mixtures with air inleak. For potential sub-ambient use of these, ignition energy sources and quenching geometries have to be considered carefully.

Hydrogen, methane and one of their fuel blends combustion: CFD analysis and numerical-experimental comparisons of fixed and mobile applications

The capabilities of Computational Fluid Dynamics (CFD) coupled with detailed chemistry simulations are examined in both steady jet diffusion flames and in an internal combustion engine case fuelled with hydrogen. Different approaches to turbulence-chemistry interaction such as the “Laminar Flame Concept” the “Eddy Dissipation Concept” and the “Turbulent Flame Speed Closure” are considered and tested. The results are compared with the experimental data available. Concerning the jet diffusion flames, the combustion processes of hydrogen, methane and one of their fuel blends are investigated on two burner geometries. Different sensitivities (i.e. mesh, turbulence model, turbulent Schmidt number, chemical mechanism) are performed. The study demonstrates that despite the burner geometry considered and the chemical composition of the fuel, the Complex Chemistry with “Eddy Dissipation Concept” is the model that better describes the behaviour of the turbulent flames. On the other hand, the “Laminar Flame Concept” sub-model is characterized by an higher fuel consumption rate, which causes an overestimation of the temperature peak. As for the in-cylinder unsteady simulations, the hydrogen combustion process is better described by the “Turbulent Flame Speed Closure” sub-model, which, unlike the other two, requires the specification of both laminar and turbulent flame speed. Despite different variations being considered, the “Laminar Flame Concept” adoption leads to an unphysically high burning rate, while the Eddy Dissipation Concept sub-model is characterized by an underestimation of the apparent heat release rate, and thus of the pressure peak inside the combustion chamber.

Evolution of Flame Displacement Speed Within Flame Front in Different Regimes of Premixed Turbulent Combustion

A transport equation for the flame displacement speed evolution in premixed flames is derived from first principles, and the mean behaviours of the terms of this equation are analysed based on a Direct Numerical Simulation database of statistically planar turbulent premixed flames with a range of different Karlovitz numbers. It is found that the regime of combustion (or Karlovitz number) affects the statistical behaviour of the mean contributions of the terms of the displacement speed transport equation which are associated with the normal strain rate and curvature dependence of displacement speed. The contributions arising from molecular diffusion and flame curvature play leading order roles in all combustion regimes, whereas the terms arising from the flame normal straining and reactive scalar gradient become leading order contributors only for the flames with high Karlovitz number values representing the thin reaction zones regime. The mean behaviours of the terms of the displacement speed transport equation indicate that the effects arising from fluid-dynamic normal straining, reactive scalar gradient and flame curvature play key roles in the evolution of displacement speed. The mean characteristics of the various terms of the displacement speed transport equation are explained in detail and their qualitative behaviours can be expounded based on the behaviours of the corresponding terms in the case of 1D steady laminar premixed flames. This implies that the flamelet assumption has the potential to be utilised for the purpose of any future modelling of the unclosed terms of the displacement speed transport equation even in the thin reaction zones regime for moderate values of Karlovitz number.

Organic and Inorganic Hybrid Composite Phase Change Material for Inhibiting the Thermal Runaway of Lithium-Ion Batteries

To deal with the flammability of PA (paraffin), this paper proposes a CPCM (composite phase change material) with a high heat-absorbing capacity for mitigating the thermal runaway of lithium-ion batteries. Two heating power levels were used to trigger thermal runaway in order to investigate the influence of heating power on thermal runaway characteristics and the mitigation effect of the PCM (phase change material). Thermal runaway processes and temperature changes were recorded. The results showed that heating results in a violent reaction of the battery, generating a high temperature and a bright flame, and the burning of PA increases the duration of a steady flame, indicating an increased threat. SA (sodium acetate trihydrate) effectively inhibited PA combustion, and the combustion time was reduced by 40.5%. PA/SA effectively retarded the rise in temperature of the battery, and the temperature rise rate was reduced by 87.3%. Increased heating power caused faster thermal runaway, and the thermal runaway mitigation effect of the CPCM was dramatically reduced. This study may provide a reference for the safe design and improvement of thermal management systems.

On the flame transfer function models for laminar premixed conical and V- flames considering the stretch effect

This paper investigates a predictive model that considers the impact of stretch on the dynamic response of laminar premixed conical and V- flames; the flame stretch consists of two components: the flame curvature and flow strain. The steady and perturbed flame fronts are determined via the linearized G-equation associated with the flame stretch model. Parameter analyses of the effects of Markstein length L, flame radius R and unstretched flame aspect ratio β are also conducted. Results show that the flame stretch reduces the steady flame height, with this effect being more significant for larger Markstein lengths and smaller flame sizes. The effects of flame stretch on perturbed flames are evaluated by comparing the flame transfer function (FTF) considering the flame stretch and not. For flames of different sizes, the impact of flame stretch on FTF gain can be divided into three regions. When both β and R are relatively small, due to the decrease in steady flame height and the impact of flow strain, the FTF gain increases. As β and R gradually increase, the FTF gain of the conical flame oscillates periodically while the FTF gain of the V-flame decreases, primarily due to the flame curvature enhancing the flame front disturbance and the wrinkle counteracting effect. When β and R are large, a disruption in wrinkle counteracting effect ensues, leading to a significant increase in FTF gain. Furthermore, as the actual flame height is reduced, the flame stretch also reduces the FTF phase lag which is related to the disturbance propagation time from the flame root to the tip.

Flash-back, blow-off, and symmetry breaking of premixed conical flames

Hydrodynamic and thermal-diffusive effects subject premixed flames to intrinsic instabilities that strongly influence their shape and propagation/stabilization characteristics. However, the interaction and coupling of intrinsic flame dynamics with background flow gradients and boundary conditions remain poorly understood. This paper presents a global nonlinear bifurcation analysis of burner-stabilized laminar premixed conical flames with varying reaction rates and reactant diffusivities, respectively parameterized by the Damköhler number Da and the Lewis number Le. Using a dimensionless formulation of the reacting, weakly-compressible Navier–Stokes equations, the dynamics of flash-back, blow-off, and cellular instability are explored in a fully-coupled framework. Our analysis identifies steady conical flame states over a finite range of Da and Le, limited by saddle–node bifurcations corresponding to spontaneous blow-off of the axisymmetric flame below a Le-dependent lower critical Da value and spontaneous boundary-layer flash-back beyond a higher critical Da value. Furthermore, the analysis reveals that the conical flame loses its axisymmetry via circle–pitchfork bifurcations as Le or Da decrease or increase beyond respective critical values. These bifurcations are shown to correspond to stationary three-dimensional global modes describing steady polyhedral or tilted flame structures, each associated with distinct azimuthal periodicities. Statement of SignificanceThis work presents a bifurcation analysis of laminar premixed conical flames within a fully-coupled flame/flow model. By considering variations in reaction rate and reactant diffusivity, the analysis identifies saddle–node bifurcations corresponding to spontaneous flash-back and blow-off of the axisymmetric flame. It also identifies axisymmetry breaking bifurcations associated with transitions to steady three-dimensional polyhedral and tilted flame states. These global instabilities indicate that hydrodynamic and thermal-diffusive effects strongly influence a flame’s steady structure in the azimuthal dimension even when no instabilities appear in the plane of symmetry. As such, future analyses of flame dynamics should rule out symmetry-breaking behaviors before adopting a two-dimensional computational framework.

Dynamic imaging of three-dimensional temperature field via three-directional wavelength modulated lateral shearing interferometry

A three-directional wavelength modulated lateral shearing interference system was proposed for the dynamic three-dimensional visualization of a temperature field. The laser emitted from a distributed feedback (DFB) laser was divided into three beams by a fiber beam splitter. The beams were spatially expanded and passed through the flame in three directions of 0°, 120° and 240°, respectively. The three interferograms formed by reflections from the front and back surfaces of the parallel glass plates were spatially converged into the lens of a high-speed camera. Multiple plane mirrors were utilized to reflect the beams. The size of the region of interest (ROI) was −20 ∼ 20 mm in both x and z directions and 5 ∼ 30 mm in y direction, with a spatial resolution of 200 μm in all directions. A three-dimensional sensitivity matrix was modeled, and the simultaneous algebraic reconstruction technique (SART) was used for layer-by-layer imaging to achieve three-dimensional flame monitoring. Numerical simulations verified the effectiveness of the algorithm qualitatively and quantitatively. In experiments, the temperature distributions of four cases of flames, namely, steady diffused flame, partially blocked diffused flame, dynamic acoustically excited flame and the flame after ignition, were reconstructed at 1k frames per second (fps). The flame structures and temporal variations of temperature distributions were clearly visualized, demonstrating the ability of the system in fast monitoring of three-dimensional dynamic temperature fields.